Hollow baseball and softball bats typically exhibit a “trampoline effect” when striking a baseball or softball. This trampoline effect is a direct result of the transfer of potential energy, which is stored in the local bat hoop mode as deformation, back to the ball in the form of kinetic energy. The trampoline effect is substantially optimized when the transfer of energy incurs minimal losses. This occurs when the ball is struck such that the strain recovery of the hoop mode barrel wall is in phase with the strain recovery of the ball. Under such conditions, maximum kinetic energy transfer to the ball may be realized.
The efficiency of this energy transfer to the ball can be measured as a coefficient of restitution (COR). The COR is determined by dividing the post impact ball velocity by the incident ball velocity, which represents the efficiency of energy transfer between the bat and the ball.
It is commonly believed that as the structural thickness or stiffness of the barrel wall is increased, in an effort to increase bat durability, the efficiency of kinetic energy transfer to the ball decreases. Thus, there is a direct relationship between barrel energy losses, due to stiffness, and performance. Barrel walls that are extremely thin typically perform well since they exhibit extremely high deformation (which is favorable for energy transfer), but they typically do not have good strength characteristics or durability. Barrel walls that are very thick, conversely, are typically very durable but do not efficiently transfer energy to the ball.
Double-wall or multi-wall bat barrels have been developed in an effort to increase barrel performance, while maintaining an overall wall thickness that provides sufficient barrel durability. Multi-walled bats expand the amount of deflection possible relative to a single-walled design by increasing the barrel compliance, specifically by reducing the hoop (radial) stiffness of the bat barrel. While multi-wall bats have generally been successful, they are typically more expensive to manufacture than single-wall bats. Thus, when budget or selling price is a controlling factor, single-wall bats may be desirable.
It was previously believed that single-wall composite bats would not perform well or be durable enough to justify investing significant time in their development. Single-wall bats have recently been developed, however, that include one or more polymer composite materials reinforced by three-dimensional fibers, such as woven or braided glass fibers. An example of a single-wall ball bat 5 including three-dimensional fibers 8 is shown in FIGS. 1 and 1A.
These three-dimensional fibers provide improved durability, relative to conventional polymer composite bats, without appreciably sacrificing performance. Single-wall composite ball bats including three-dimensional reinforcement fibers are, however, relatively complicated and expensive to manufacture. Thus, a need exists for single-wall composite ball bats that can be constructed using inexpensive, high volume process methods.